The present invention relates to an intake control system for use in an internal combustion engine, and an intake control valve disposed in an intake pipe and controllable by the intake control system for controlling the flow of an air-fuel mixture in the intake pipe.
When an intake stroke of an engine is started, a burned gas tends to flow from an engine cylinder and an exhaust passage back into an intake passage due to the overlapping operation of intake and exhaust valves, lowering the volumetric efficiency of the engine cylinder. Various devices have heretofore been proposed to increase the volumetric efficiency of internal combustion engines by shutting off the backflow of an air-fuel mixture from the engine cylinder during a final period of the interval in which the intake valve is open. As disclosed in U.S. Pat. No. 4,422,416, one such device comprises a reed valve disposed in an intake duct for preventing the reverse flow of an air-fuel mixture. Another deice comprises a check valve disposed in an intake duct and actuatable by the cam of an associated intake valve, as disclosed in U.S. Pat. No. 4,363,302.
Operation of the disclosed reed valve is governed solely by the intake pressure around the reed valve. Operation of the disclosed check valve is determined solely by the operation of the intake valve cam. Although the air-fuel mixture backflow can be prevented to a limited extent from taking place by those prior intake control valves, it has not been possible to completely prevent the air-fuel mixture backflow by opening and closing the intake passage depenent on operating conditions of the engine, so that the volumetric efficiency will be kept at a maximum.
More specifically, a valve overlapping period in which the intake and exhaust valves are simultaneously opened is determined solely by the cams which open and close the intake and exhaust valves. If such a valve overlapping period is selected to maximize the volumetric efficiency in a low engine speed range, then the volumetric efficiency in a high engine speed range is lowered. Conversely, if the valve overlapping period is selected to maximize the volumetric efficiency in the high engine speed range, then the volumetric efficiency in the low engine speed range is lowered. To eliminate the above shortcoming, it has been proposed to achieve a maximum volumetric efficiency throughout all engine speed ranges by selecting the valve overlapping period to be equal to the period in which the volumetric efficiency in the high speed range is maximized, and which is shorter than the corresponding period in the low speed range. In the low speed range, the timing to open the intake passage is delayed by the intake control valve. Therefore, the valve overlapping period is varied substantially dependent on the rotational speed of the engine. To accomplish the above control process, an intake control valve capable of quickly opening and closing the intake passage is required. The conventional intake control valves have failed to meet such a requirement since they cannot be opened and closed with sufficient response.
The torque vs. speed curve of a commercially available internal combustion engine is indicated by the solid line in FIG. 16 of the accompanying drawings. As shown in FIG. 16, the torque of the engine is smaller in a high speed range than the torque (indicated by the dotted line) of an engine in which cams designed for high-speed operation are employed, and is smaller in in a low speed range than the torque (indicated by the dot-and-dash line) of an engine in which cams designed for low-speed operation are employed.
In view of the above problem, there have been proposed various valve control mechanisms for varying the lift timing of an intake or exhaust valve according to operating conditions of an engine, such as a rotational speed thereof. One proposal is disclosed in U.S. Pat. No. 4,347,812. In the prior valve control mechanisms, the lift timing itself of an intake or exhaust valve is varied according to operating conditions of the engine.